A method for detecting a phase shift in motor driving and a motor using the method as described above are conventionally disclosed in Patent Literature 1, for example. In a brushless motor that includes magnetic pole position sensors for detecting a rotation position of the rotor, for example, the misaligned mounting position of a magnetic pole position sensor causes an error in the detection position. Such an error in the detection position hinders precise rotation control of the brushless motor, thus reducing the rotation efficiency, for example.
To address this problem, the method disclosed in Patent Literature 1 determines an error in the alignment of a magnetic pole position sensor by calculating a phase difference between a sensor signal from the magnetic pole position sensor and a back EMF signal including a local minimum value from the motor drive circuit. That is, first, a brushless motor is coasted in the state where the motor driving is stopped. Next, during this coasting period, a sensor signal and a back EMF signal are measured. Then, the phase of the local minimum value of the back EMF signal and the phase of the pulse changing point of the sensor signal are obtained, and an error in alignment is determined based on the difference between these phases. Further, by correcting the sensor signals based on accumulation of these phase differences, the detection position in the rotor is corrected.
Another method is proposed so as to correct the energization timing by detecting the amount of phase shifts from the ideal position, using a comparator (see Patent Literature 2, for example). That is, the technique disclosed in Patent Literature 2 detects the induced voltage in the W phase using the comparator, and detects the phase difference between the output signal from the comparator and the detection signal from a Hall element in the U phase.
In the conventional method disclosed in Patent Literature 1, the phase of the local minimum value of the back EMF signal needs to be obtained. However, such a local minimum value changes depending on the amplitude of the back EMF signal, and the difference between the peak value of the back EMF signal waveform and the local minimum value is small. This poses a limitation in accurately determining the phase of the local minimum value in a simple manner.
In the method disclosed in Patent Literature 2, in the process of converting the induced voltage into pulses, a delay caused by hysteresis of the comparator makes an error in detection of the phase. Further, because the time delayed by the hysteresis is affected by the amplitude and frequency of the induced voltage, accurate correction of the delay time is difficult from a practical point of view.
PTL1    Japanese Patent No. 4,191,172
PTL2    Japanese Patent Unexamined Publication No. 2009-240041